Electrodialysis is a well known art (see U.S. Pat. Nos 4,325,792, 4,439,293, 4,636,288, 3,394,068, 4,080,270, 4,111,772, 4,203,822, 4,519,881 the disclosure of which are incorporated by reference). Electrodialysis is the electrotransport of ions through ion permeable membranes as a result of an electrical driving force. The process is commonly carried out in an electrochemical cell having a catholyte compartment containing a cathode and a catholyte and an anolyte compartment containing an anode and an anolyte, the catholyte and anolyte compartments being separated by ion permeable membranes.
The electrotransport of sodium and other alkali metal cations through cation permeable membranes is a well known art. The electrotransport of multivalent metal cations through cation permeable membranes into an electrolyte that comprises agents, including hydroxyl ions, that insolubilize multivalent metal cations, is disclosed in U.S. Pat. No. 4,636,288. The multivalent metal cations were soluble in the aqueous acidic solution and the cations were sufficiently mobile for electrotransport through a cation permeable membrane. The electrodialytic processes of U.S. Pat. No. 4,636,288 could be operated either as a batch process or as a continuous process with little difference in the results. Conversion of the multivalent metal cation salts resulted in forming the acid of the salt anion and the base of the salt cation. The electrical conductivity and acidity of the solution comprising the multivalent metal salts increased with increasing conversion. In contrast to acidic solutions of multivalent metal salts, multivalent metal cations in alkaline solutions are substantially insoluble hydroxides, the multivalent metal cations are not generally ionically mobile, the hydroxyl ion does not form an ionically conductive acid and the solution becomes essentially non-conductive electrically when the alkali hydroxide is depleted.
Many of the hydroxides of heavy metals, such as aluminum, lead, tin, zinc, gallium and tungsten are soluble or appear to be soluble in excess of sodium or potassium hydroxide. This has been attributed to the formation of salts, the hydroxides behaving as amphoteric substances and giving either OH.sup.- or H.sup.+ ions according to the condition of the experiment. For example, when aluminum hydroxide is dissolved in sodium hydroxide, sodium aluminate is supposed to be formed. It is possible, however, that the solution of the aluminum is not so much a matter of compound formation as a peptization of the hydroxide to form a sol, gel or colloidal dispersion. It is of course, difficult to mechanically separate the multivalent metal hydroxide from an alkali hydroxide gel, sol, solution or colloidal dispersion.
When a metal has several oxides, the basic properties of the hydroxides become less pronounced as the valency of the metal increases. When a certain limit of valency is reached, the basic properties disappear almost completely and salt formation does not take place to an appreciable extent. The acidic oxides are formed only by those metals which can exert a high valency and thus combine with several oxygen atoms. The acidic tendency is almost invariably in the unequivalent and higher valence of the metal. Hence metals in the right hand half of the periodic table give acidic oxides that form salts with alkali metal cations. These acids or multivalent metal hydroxides, metallic oxides-acidic, such as molybdic, tungstic, uranic, vanadic niobic and tantalic, cannot always be isolated in pure form by neutralization of the salts as they are frequently converted to anhydrides or polymerized or dissolved in excess of the neutralizing acid. These acids are made from a lower oxide of the metal by heating the oxide with alkali, usually in the presence of an oxidizing agent. The excess alkali is removed by neutralization with an acid to form a soluble alkali salt.
In many uses of alkali hydroxides, excess of the alkali hydroxide is used, resulting in the formation of gels, sols, dispersions and solutions. This is especially the case for sodium hydroxide etchants of aluminum and alloys of aluminum. Aluminum is usually etched in a 15% to 30% solution of sodium hydroxide at temperatures above 80 C until about 20 to 30 grams of aluminum is etched per liter of etchant. The aluminum appears to be dissolved and seeding and cooling results in a very limited removal of sodium aluminate or aluminum hydroxide by filtration of the etchant. Since the etching rate decreases as the aluminum is etched and then accumulates in the sodium hydroxide solution, it is necessary to replace the etchant to maintain relatively 1o concentrations of aluminum. The alloys of aluminum contains copper, silicon and other metals that are etched into the sodium hydroxide etchant and form a smut or slime or solution. To facilitate the etching process additives of sulfur, amines and wetting agents are used in the sodium hydroxide etchant. It would be desirable that the etchant be continuously restored to maintain a desired milling rate and quality of the milled alloys. It is an objective of the instant invention to provide a continuous process for purification and restoration of alkali hydroxide etchants used for etching aluminum and aluminum alloys.
It is known that the alkali hydroxide etchants can be neutralized or acidified with an acid, such as sulfuric, and thereby the aluminum hydroxide separated by filtration. The resulting salt solution can be electrodialytically converted to the acid of the salt anion and hydroxide of the alkali cation as disclosed in U.S. Pat. No. 4,636,288. The acid could be used again to neutralize the etchant and the alkali hydroxide returned to the etchant. This two step process requires that all of the alkali cations be converted to a salt and then the salt be electrodialytically converted to the alkali hydroxide for return to the etchant and that all additives must be recovered in the electrodialytic salt conversion step. The filtrate, salt, solution, obtained in removing the aluminum hydroxide, contains calcium, magnesium, and other multivalent metal cations that must be removed from the filtrate to prevent fouling of the cation permeable membranes when electrodialytically processing the filtrate or an electrodialytic well having three or more compartments must be used. It would be preferable that the alkali hydroxide etchant be restored in a one step process whereby the unwanted aluminum and other metal hydroxides are continuously separated from the etchant and the alkali hydroxide and additives returned to the etchant. It is an object of the instant invention to provide a continuous electrodialytic process for purification and restoration of alkali hydroxide etchants of aluminum.
The elctrodialytic conversion of salts of multivalent metal cations in aqueous solutions with or without admixture with salts of monovalent metal cations comprises the electrotransport of multivalent metal cations through cation permeable membranes whereby the multivalent salts are converted to the acids of the salt anions and insoluble hydroxides, salts or substances of the multivalent metal cations. The electrotransport of a multivalent metal cation is facilitated by the use of a salt of an acid in the electrolyte into which the multivalent metal cation is electrotransported.
When electrodialytically converting mixtures of alkali and multivalent metal salts, the multivalent metal cations are electrotransported from an electrolyte to a reactor electrolyte where the multivalent metal cations are ionically immobilized and the alkali cations are electrotransported from the reactor compartment to a catholyte of an aqueous solution of an alkali hydroxide. Although the process disclosed in U.S. Pat. No. 4,636,288 provide a means for separating multivalent metal cations and alkali cations, they do not provide a means for electrodialytically separating multivalent metal hydroxides or metallic oxides-acidic as removable solids from a feed compartment electrolyte when the feed comprises an alkali hydroxide or a method for electrodialytically separating multivalent metal hydroxides.
It is a feature of this invention to provide an electrodialytic process for separating multivalent metal hydroxides from alkali hydroxides and the electrodialytic separation of multivalent metal hydroxides whereby a multivalent metal hydroxide is separated as a removable solid in an electrolyte and a multivalent metal cation is electrotransported from the electrolyte.